SEROUS AND SYNOVIAL MEMBRANES. 231 



teguments that glide over them, or amongst the tendons themselves, appear to 

 be essentially similar to the synovial membranes in the arrangement of their 

 elements ; but their epithelium is less regular, and shows numerous gradations 

 of cell-growth. The fluid of the Serous sacs is normally only sufficient to keep 

 their surfaces moist, and its composition in the healthy state is consequently in- 

 determinable. In various abnormal conditions, however, it accumulates in large 

 amount ; and as this may occur from simple obstruction to the venous circulation, 

 without any morbid affection of the membrane itself, it is probable that the fluid 

 which thus transudes is very similar in quality to the natural serous exhalation. 

 As a general rule, the fluid effused from Serous membranes resembles the serum 

 of the blood with a considerable proportion of its albumen kept back, the salts 

 being present in nearly their normal amount ; the amount of albumen present, 

 however, is subject to great variation, but the recent researches of Schmidt and 

 Lehmann 1 have shown that it presents a remarkable degree of constancy in the 

 exudations from each membrane. Thus, the transudation of the pleura contains 

 about 2.85 per cent, of albumen; that of the peritoneum only 1.18 per cent., that 

 of the arachnoid no more than 0.6 or 0.8 percent., and that of the subcutaneous 

 areolar tissue as little as 0.36 per cent. There is strong reason to believe that 

 the retention of the chief part of the albumen, when the water and the salines 

 transude the coats of the vessels, is merely the result of the physical arrangement 

 of the elements of the membranes ; it having been shown by Valentin, 3 that the 

 filtration of albuminous fluid through dead serous membranes is attended 

 with the same result. And it can scarcely be doubted, therefore, that the varia- 

 tion in the quantity allowed to pass by different membranes, is to be attributed 

 to their physical peculiarities. In proportion to the increase of the pressure to 

 which the blood may be subjected (as through an obstruction to its return, or any 

 other cause), is the increase in the proportion of albumen which transudes; and 

 in some cases of extreme obstruction, without inflammation, the presence of soft 

 jelly-like masses or of strings of fibrin, indicates that even this component of the 

 blood may be made to transude by a further augmentation of pressure. 3 The 

 fluid of the Synovial capsules and of the Bursa3 is of a much more viscid charac- 

 ter, almost resembling oil in its glairy appearance, and not mixing readily with 

 other liquids ; its composition has not been precisely made out ; but it certainly 

 contains a far larger proportion of albuminous matter than the serous exudations, 

 and it may be probably considered as a true product of secretion. The pur- 

 pose of all these fluids is obviously to diminish friction between surfaces which 

 are exposed to mutual attrition ; and the quantity of albumen they respectively 

 contain seems to have reference to the amount of motion and of pressure to which 

 the membranes are subjected, being least in the cavities of the brain, somewhat 

 more in that of the peritoneum, two and a half times as much in that of the 

 pleura, and many times greater in the synovial capsules. It is probable that the 

 rate of nutrition of the Serous membranes is not rapid under ordinary circum- 

 stances ; since there appears to be but little vital activity in them. Their epi- 

 thelium exhibits no indications of being frequently cast off and renewed, like 

 that of many other parts ; and is probably very permanent in its character. The 

 membranes of the Synovial capsules and of the Bursae, however, obviously possess 

 a much higher vital activity, being themselves more vascular, and having an 

 epithelium which is evidently in continual course of renewal ; and this activity 

 seems connected with their secretory office. All these membranes are very 

 readily regenerated after loss of substance ; and they are even produced de novo, 



1 " Lehrbuch der Physiologischen Chemie," band ii. pp. 248-50. 



2 " Lehrbuch der Physiologie," band i. p. 601. 



3 This has been experimentally demonstrated by Mr. Robinson, who has shown that the 

 urine may be rendered albuminous or even fibrinous by the application of a ligature round 

 the renal vein. (" Medical Gazette," June 28, 1844.) 



